Project Summary Posttranslational modifications (PTM) regulate protein function but also create new self-antigens that may be responsible for the development of autoimmune diseases. Type 1 diabetes (T1D) and other autoimmune endocrinopathies have recently been shown to arise in cancer patients treated with various checkpoint inhibitors (CPIs; anti-CTLA-4, anti-PD1 and anti-PDL1). However, it is not known if these autoimmune adverse events are triggered by molecular events or pathologic mechanisms similar to those leading to classical autoimmune diseases. Our preliminary data suggest that most CPI-treated patients who developed diabetes express MHC class II HLA-DR4 and therefore differ from spontaneous type 1 diabetes (T1D) patients who favor HLA-DR3. Thus early HLA studies suggest that different forms of self-antigens may trigger CPI-induced vs. classical T1D. HLA-DR4 is associated with rheumatoid arthritis (RA) by binding to citrullinated PTM peptides and, in a similar manner, CPI-induced diabetes may also involve citrullinated self-antigens. Supporting this hypothesis, our preliminary data suggest that CPI-treated patients who developed diabetes show a specific increase in anti-citrullinated glucokinase autoantibodies in their serum compared to other CPI- treated patients and display elevated frequencies of autoreactive nave B cells that express unmutated antibodies binding citrullinated glucokinase. Our research goals are to determine how PTM may favor the development of autoimmunity. Our working hypothesis is that CPI induce an increase in autoreactive B cells that recognize citrullinated and other PTM self-antigens produced by stressed pancreatic beta cells. These B cells bind and present PTM self peptides through HLA-DR4 to activate autoreactive T cells, responsible for the development of diabetes. The present studies will therefore identify PTM proteins in pancreatic b cells that arise in both CPI-induced T1D and spontaneous T1D and serum autoantibodies targeting PTM self-antigens. We will also characterize autoreactive B and T cells in patients with CPI-induced and spontaneous diabetes. Understanding the fine specificity of B and T cell responses will reveal pathways leading to a break in tolerance to PTM self-antigens, which constitute novel biomarkers of b cell biology in both drug-induced and spontaneous T1D.